Cold rolled steel sheets and nonoriented electromagnetic steel sheets and the like rolled so as to have a final thickness are usually subjected to final annealing at a high temperature in a reductive atmosphere and then coated according to needs, thereby obtaining final products. There are many types of paints, and a water-based paint containing an organic resin is usually used. Furthermore, there are many types of coating processes, and a roll coating process is widely used because this process is satisfactory in productivity and fit for strictly controlling the thickness of thin films. In a process using such water-based paint, a coating liquid is applied onto a steel sheet and the resulting steel sheet is heated, thereby drying the applied liquid and then baking the obtained coating. In known processes, a heating apparatus such as an air(gus)-heating furnace and electric furnace are used because such furnaces are relatively low in equipment cost and operating cost.
Recently, in view of productivity, demands have been made on a high-speed coating process. For example, Japanese Unexamined Patent Application Publication No. 11-262710 discloses a coating apparatus that can be operated at a line speed of 150 m/min. In known heating methods, however, there is a problem in that rapid heating operations are difficult and seriously uneven coatings are formed.
In order to cope with such a problem, for example, Japanese Examined Patent Application Publication No. 53-4528 discloses a process for manufacturing a coated steel sheet. The process includes a step of applying a coating liquid onto a steel sheet, a step of heating the resulting steel sheet for 1-5 seconds by an infrared radiation method, and a step of baking the resulting steel sheet at high speed by a high-frequency induction heating method, wherein these steps are performed in that order.
On the other hand, Japanese Unexamined Patent Application Publication No. 3-56679 discloses another process for manufacturing a coated steel sheet, because moisture cannot be sufficiently removed from the coating liquid using radiation heat, whereby the following defects are caused: appearance defects such as orange peels and poor film properties such as poor adhesion. This process at least includes a drying step (the heating temperature is about 130 to 150° C.) in which the heating rate is 20° C/s or less and a high-frequency induction heating method is used. Furthermore, Japanese Unexamined Patent Application Publication No. 62-133083 discloses other processes including a drying step using a high-frequency induction heating method and a subsequent heating step using an air(gas)-heating furnace.
However, even if the above processes are used, fine uneven portions are formed in coatings and therefore it is difficult to avoid the formation of uneven coatings on an industrial scale. Further, a coating appearance defect called flash rust arises in some cases depending on the composition of a coating liquid, and such a defect cannot be sufficiently eliminated by the above processes.
Since there are the above-mentioned problems, the operating speed of current drying and baking lines is usually about 60-80 m/min and, even in contemporary lines, is 150 m/min at the most.
In recent years, in coating steps, coating lines are directly connected to final annealing furnaces. Therefore, in order to avoid an increase in length of lines for manufacturing steel sheets, there are needs for compact coating lines. For such a purpose, vertical coating lines (steel sheets are subjected to coating, drying, baking, and the like while they are moved in substantially a vertical direction) are preferable as compared with known horizontal coating lines (steel sheets are subjected to coating, drying, baking, and the like while they are moved in substantially a horizontal direction), because the horizontal coating lines occupy a large area. However, during this research, we found that the above-mentioned coating unevenness is serious when such vertical coating lines are used.
Current coating processes have another following problem: when a water-based coating liquid containing a resin is continuously applied onto steel sheets for a long time using a roll coater coating apparatus directly connected to a final annealing furnace, the heat of the steel sheets causes the resin to be adhered to the roll coater and therefrom coating appearance defects arise.
In order to solve such a problem, Japanese Unexamined Patent Application Publication No. 4-154972 discloses a process for forming a coating on an electromagnetic steel sheet. In this process, when a treating liquid containing a chromium compound and an organic resin is applied onto such an electromagnetic steel sheet processed in a final annealing step, the treating liquid and electromagnetic steel sheet are maintained at 25° C. or less.
According to the process, the resin can be prevented from being adhered to the roll coater when the treating liquid and steel sheet are maintained at 25° C. or less. However, this advantage is limited. Even if the process is employed, in a long time, the resin is adhered to the roll coater depending on the type of the resin.
Coated steel sheets include nonoriented electromagnetic steel sheets coated with an insulating film formed by a painting method. When such nonoriented electromagnetic steel sheets are manufactured by the above manufacturing processes, the problems below arise.
The coated nonoriented electromagnetic steel sheets are used for iron cores for motors and transformers in many cases, and the iron cores are prepared according to the following procedure: each steel sheet is punched into pieces having a predetermined shape by a punching process and the obtained pieces are stacked. Therefore, the steel sheets must have satisfactory punchability and weldability properties (for welding end faces). In order to enhance the punchability, it is effective that the insulating film contains a resin, that is, such a resin is a component (coating component) of the insulating film. However, the contained resin causes blowholes during a welding operation. Therefore, it is necessary to obtain both satisfactory punchability and weldability.
In order to achieve the compatibility between the punchability and weldability of the nonoriented electromagnetic steel sheets, the processes below have been proposed.
(1) A process in which roughness is increased on a steel sheet or an insulating film (for example, Japanese Unexamined Patent Application Publication No. 60-190572)
(2) A process in which an insulating film contains Al (for example, Japanese Unexamined Patent Application Publication No. 9-291368)
(3) A process in which a resin is improved in heat resistance (for example, Japanese Unexamined Patent Application Publication No. 6-235070)
(4) A process in which a double layer coating is used (for example, Japanese Examined Patent Application Publication No. 49-6743)
(5) A process in which a liquid containing a chromate-based inorganic coating component and a resin component is applied onto a steel sheet and a special resin is concentrated at the surface of a coating (for example, Japanese Examined Patent Application Publication No. 4-43715)
In process (1), although satisfactory punchability and weldability can be obtained, magnetic properties of an obtained core material are inferior because stacked steel sheet pieces have a small space factor. In processes (2) and (3), there is plenty of room for further improvement because the compatibility between the following properties cannot be achieved: superior TIG weldability equivalent to those of an inorganic coating and superior punchability equivalent to those of an organic coating. In process (4), there is a problem in that manufacturing cost and the like are high because a procedure of applying a coating liquid onto a steel sheet and then baking the resulting steel sheet is repeated twice, that is, two coating operations and two baking operations are performed. In process (5), there is also a problem in that manufacturing cost is high because available resins and inorganic components are limited.
Accordingly, in the known coating processes, the compatibility between the satisfactory punchability and weldability cannot be achieved without causing other serious problems.
A species of nonoriented electromagnetic steel sheet delivered in the semi-processed state has the problems below.
A process for manufacturing an electromagnetic steel sheet includes the following subsequent steps:                (a) a step of forming a steel ingot, such as a slab, having adjusted composition,        (b) a step of hot-rolling the slab and then annealing the hot-rolled steel sheet according to needs,        (c) a step of subjecting the steel sheet to cold-rolling (or warm-rolling) and then subjecting the resulting steel sheet to annealing, once or several times according to needs, and        (d) a step of providing an insulating film on the resulting steel sheet according to needs (insulating coating treatment).        
A process for manufacturing a nonoriented electromagnetic steel sheet delivered in the semi-processed state further includes a step of temper-rolling the resulting steel sheet to apply a strain to the steel sheet in addition to the above steps, wherein the temper-rolling step follows step (c). Step (d) of providing the insulating film is then performed according to needs.
The temper-rolling step may follow step (d) for performing the insulating coating treatment in order to avoid increasing the complexity of handling when an annealing apparatus (usually a final annealing apparatus) used in the final part of step (c) is directly connected to an apparatus for the insulating coating treatment and therefore a temper rolling mill cannot be installed therebetween. In this case, there is a problem in that the insulating film is partly damaged during the application of strain and thereby the film properties are deteriorated.